1. Field of the Art
The present invention relates to an image forming method, and more particularly to an image forming method wherein an electron beam irradiates an object with a scanning electron microscope, and secondary electrons from the object are used to form an image of the object.
2. Prior Art
In measuring a pattern of a semiconductor integrated circuit, an image of the pattern is formed using a scanning electron microscope (hereinafter abbreviated as SEM). It is known that various types of noises are generated while forming an image with SEM so that the image is superimposed with noises. Such noises include distribution noises of a primary beam from SEM, and noises generated during secondary electron emission. The former noises are generated because a fraction of the electron beam emitted from a cathode and running within the column of the SEM are captured by an intermediate aperture with a certain probability before they reach the surface of an object. The latter noises are generated because the physical phenomenon of secondary electron emission is essentially a phenomenon based on probability. Other noises may be generated from components such as a photomultiplier, A/D converter and the like in the measuring system.
In order to form an image not susceptible to such noises, the following methods have been adopted heretofore:
(1) eliminating high frequency noises of an image with a low-pass filter;
(2) scanning an object plural times to use an addition and averaging processing;
(3) smoothing an image with a spatial filter processing;
(4) using a median value of pixel intensities of an image within a predetermined area through median filtering processing; and
(5) performing a frequency analysis of an image through fast Fourier transformation.
With the above methods, noises are not eliminated sufficiently to form a clear image because of the following reasons.
(1) First, the method of using a low-pass filter poses a problem that it is very difficult to set an optimum cut-off frequency. The frequency of noises on a SEM image varies with an object. Since a semiconductor integrated circuit has a number of image objects, it is very difficult to set an optimum cut-off frequency for all the image objects.
(2) With a conventional method of using an addition and averaging processing, proper noise elimination is not possible for the case where noises are continuously generated at a certain fixed frequency.
(3) The smoothing method poses a problem that the image quality after smoothing is dependent on the values of spatial filtering matrix and the quantitative evaluation of an image is affected. It also has a problem of lowering the contrast of an image.
(4) The method of using median filtering processing has been used as a technique of preserving information on pattern edge. This method is not so much effective for an image forming system wherein information whose dimension is substantially the same as the beam diameter of SEM is superposed on noises.
(5) Fourier transformation can sufficiently eliminate noises. However, a presently available processor used for image processing is not so fast as to sufficiently perform Fourier transformation.